Evaluation of the Performance of Green Roof Substrates with Recycled Materials: A Three-Year Comparative Study

The aim of this study was to test newly developed green roof substrates with a significant content of recycled materials under real conditions and to compare them with a commercially available substrate.

A two-layer extensive green roof of 7×5 m² was constructed in 2020 and divided into four sections, two of which had top layers based on new substrates. These two substrates contained the same amount of crushed brick from demolition waste (37.5% by volume) but differed in the amount of pyrolyzed sewage sludge biochar (9.5% by volume in one and none in the other). The commercial substrate was mostly based on expanded shale, lava, and pumice. Hydrophilic mineral wool was used as the bottom layer of the green roof system to improve the water retention layer. Vegetation was established with sedum carpets.

Undisturbed substrate samples were taken in 2021, 2022 and 2023 to monitor changes in hydrophysical properties (retention curves, saturated hydraulic conductivity, grain size). At the same time, vegetation development over time was monitored visually, and substrate temperature and humidity were continuously measured by autonomous sensors.

Plants in the biochar and demolition debris plots rooted faster into the substrate and achieved higher cover. While plants in plots with commercial substrate or without biochar turned red in response to stress during periods of lower rainfall or more extreme temperatures, plants in the biochar-containing plot remained lush green longer. In the following year, a greater number of emergent plants (primarily grasses) that spread from the surrounding area were observed on the biochar-amended substrate. This was thought to be due to the increased availability of nutrients from biochar.

Surface temperature amplitudes were higher than substrate and mineral wool temperatures, locally influenced by the plant biomass surrounding the sensors. Temperatures of the substrate and hydrophilic mineral wool were more stable. Differences in substrate temperatures were observed particularly between substrates containing recycled materials and the commercially available substrate.